1. Field of the Invention
This invention relates to a process for producing vitamin A aldehyde (retinal).
Vitamin A aldehyde is not only a valuable biologically active substance as itself but also a useful starting material for producing carotene.
2. Description of the Prior Art
As a simple and easy production process of vitamin A aldehyde, there is known a process oxidizing vitamin A obtained by the hydrolysis of commercially available vitamin A acetate. However, vitamin A or vitamin A aldehyde is thermally unstable because they have not only a number of double bonds sensitive to oxidation but also highly reactive allyl alcohol or .alpha.,.beta.-unsaturated aldehyde structures, and further they must be handled under a condition cut off light and oxygen.
Accordingly the oxidation of vitamin A and vitamin A aldehyde must be carried out under a mild condition. Also the reaction products tend to decompose under a severe condition of after treatment resulting in a poor yield of objective material. Further, vitamin A has all double bonds in trans form, so that it is required to be transformed into vitamin A aldehyde having all double bonds in trans form unchanged without isomerization.
Under these restricted oxidation conditions, several methods have been disclosed as described below. For example:
(1) An oxidizing method using manganese dioxide (R. A. Morton et. al., Biochem. J., 42, 516 (1948)) or nickel peroxide (Ger. Offen. 2,415,928 (1973))
(2) An oxidizing method by oxygen in the presence of platinum catalyst (Karrer, et. al., Helv. Chim. Acta 40, 265 (1957))
(3) An oxidizing method by oxygen in the presence of 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl and copper chloride catalyst (Japanese Patent Application Laid-open No. 63-233943)
(4) An oxidizing method by acetaldehyde in benzene in the presence of aluminum isopropoxide Al[OCH(CH.sub.3).sub.2 ].sub.3 (Hawkins, et. al., J. Chem. Soc., 411 (1944)).
These prior art methods have the following problems. The method (1) requires a large excess amount of solid metal oxide relative to vitamin A. And also the yield by the mothod (1) depends on the activity of the metal oxide. The method (2) requires an expensive platinum catalyst, and also has a lower yield. The method (3) requires expensive 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl as a catalyst and also a solvent having a higher boiling point such as N,N-dimethylformamide. The method (4) requires a severe reaction condition resulting in a lower yield.
An object of the invention is to provide a process for producing vitamin A aldehyde from vitamin A economically and in high yield without isomerization.